Ostraka is a general term used to describe flakes of stone or pieces of pottery used as writing or drawing material. It is also extended to cover potsherds used for the same purpose. The Egyptians used ostraka for all sorts of purposes: keeping work records, making sketches of scenes and plans, practising writing, or simply for doodling. At sites such as Deir el-Medina and the Valley of the Kings they have been found in their thousands.
The examples from TT99 are somewhat more modest. One pharaonic example bearing hieratic writing has been found, which probably dates to the 18th dynasty and contains a list of names; could they be workers on the tomb?